Haswell is the oldest CPU that supports AVX2, DDR4, quad-core standard, probably has USB3.0 slots and PCIe 3.0.
"Modern" machines like Skylake have increased to 6-core standard (even for an i5) or more (i7 and AMD Zen are 8-cores now). Those extra cores help with compiles / 3d blender renders / some other tasks, but its not a major improvement otherwise. USB 3.1 Type C isn't a big change from USB 3.0, and PCIe 3.0 and DDR4 are still mainstream.
In as little as a year however, things seem to be changing. Intel Icelake will be adding AVX512 to the mainstream later this year. We'll see whether or not consumers get an uptick in AVX512 usage, or if Adobe / other programs continue to push GPGPU instead.
With DDR5 being released in 2019 (with doubled bandwidth compared to DDR4) and PCIe 4.0 beginning adoption, and with PCIe 5.0 and USB4 standards being released, it seems like a major upgrade cycle will occur in the 2020 to 2021 timeframe. (It seems like the Desktop market is skipping PCIe 4.0 and just going straight to PCIe 5.0, quadrupling I/O bandwidth in one jump)
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Laptops are still waiting for LPDDR4 support. Kinda sucks for them. Intel really feels like they stalled out with all of these 10nm issues they've been having. A lot of their technology (ie: Ice Lake) was delayed due to the 10nm issues.